My 300D 1983 will be cooling the passenger compartment just fine when all of the sudden the heater will blow HOT Hot! This can be brought on by turning the temp knob toward warmer or sometimes it just happens on its own. I have jumpered the water valve which cuts the water off to the heater coil and things get cool again. Help Help!
Sometimes hot in Dallas.

My first thought was the monovalve needing a rebuild. But, I've never seen a monovalve be intermittent. If you take off the four screws and the valve is not flooded with coolant, it's probably not your problem, but it would certainly be my first suspect.

Can you somehow set up an indicator light or voltmeter to be viewed inside the passenger compartment while driving? Then you could see if the valve is getting a signal when the air blows hot. If the signal corresponds with the hot air, you can then go toward the amplifier as the culprit. If not the monovalve is probably at fault.

Good luck,

What is this "amplifier"? I replaced the monovalve (I didnt need to)and when I jumper the monovalve it blows cold again. I feel whatever sends the voltage to the valve is the problem.

I've never had to did that deep into the climate control on a 123 car. On the 124 cars, the amplifier is integral with the pushbutton control unit.

I think your best bet for this information would be the CD or a factory manual.

Best of luck,

Thank you. I will check into replacing the unit. DB